Multi-layer nanoparticle assemblies have been recognized as being beneficial in various industrial applications, for example, fuel cell applications, electronics, cell biology, biomedicine, and pharmaceuticals. Referring to FIG. 1 (Prior Art), a conventional assembly comprises a substrate 10 (for example, a conductive gold substrate), a cationic layer 20 (for example, polyacrylamide) over the substrate 10, and an anionic layer 30 comprised of nanoparticles (for example, silica nanoparticles) that bind to the cationic layer 20. The substrate 10 may comprise multiple cationic 20/anionic 30 bilayers over the substrate 10.
By adding the silica nanoparticles, the coated substrate is made hydrophilic, which may be beneficial for various industrial applications such as fuel cells; however, the coating may cause the substrate to lose its conductivity. In the above example, the hydrophilicity of the coated substrate is demonstrated by an advancing contact angle of 16° and a receding contact angle of 9° whereas an uncoated substrate demonstrates (more hydrophobic) advancing and receding contact angles of 84° and 18°, respectively. However, an electrical contact resistance of 174 mΩ-cm2 was measured at a contact pressure of 200 psi for the coated substrate compared to a contact resistance of 23 mΩ-cm2 for the bare substrate at the same contact pressure, thereby demonstrating that the silica nanoparticles are electrically insulating, not electrically conductive. The present inventors have recognized the importance of achieving both conductivity and hydrophilicity, especially in industrial applications such as PEM fuel cells. Consequently, the multi-layer nanoparticle coating of the present invention is optimized to achieve both of these properties. More specifically, the nanoparticle coating may be applied from a single nanoparticle suspension, eliminating the need for immersion of the piece to be coated into multiple suspensions.